1. Field of the Invention
This invention relates generally to photoelectric photometers and, more particularly to unicell photometers with an optical element for providing the photometer with different horizontal and vertical specific acceptance angles.
2. Background of the Prior Art
The use of unicell photometers for evluating scene brightnesses to predict and/or control photographic exposures is well known. Such photometers generally consist of a photoresponsive transducer and a lens element for receiving radiation from a scene and directing it onto a photosensitive surface of the transducer. The transducer, which may be either of the photovoltaic or photoresistive type, provides an output signal indicative of the radiant power incident to its photosensitive surface. The output signal from the transducer may be utilized directly to predict a photographic exposure condition or to actually control a camera's shutter and aperture settings by automatically adjusting them for the correct exposure without intervention by the photographer.
The output signal from such photometers characteristically responds in a directional manner to the influence of radiation from the various scene objects which may occupy its field of view as based on their angular field position. Consequently, the contribution which any object makes to the strength of the output signal depends, in part, on the importance or weight which is assigned to particular angular field locations by the photometer's directional response characteristic. As an example, the so-called center weighted light meter places more emphasis on objects located near the center of the picture field than on those located near its edges. Thus, in this case, a bright object will affect the output signal more when it is located on-axis than when that same object is located at some off-axis angular field position.
Angular directional response characteristics for such photometers differ one from another depending on the particular arrangement of optical elements with the photoresponsive transducer. To determine the angular directional response characteristic for a specific photometer, it is conventional to measure variations in its output signal in response to a constant output luminance source that occupies a small angular percentage of an otherwise totally dark field of view as the source is positioned at different angular field locations. From this type of measurement, it is possible to determine a generalized mathematical function which describes the relationship between the photometer's on-axis and off-axis responses. Typically, the unicell photometer has an angular directional response function which resembles a "bell-shaped" curve having a peak on-axis response which gradually decreases to zero at the edge of its field of view. For photometers which have "bell-shaped" response curves, it is apparent that the more gradual the decrease off-axis, the less the photometer will emphasize centrally located objects.
Although the angular directional response curve provides a great deal of useful and precise information about the directional performance of such photometers, it is sometimes more convenient to specify a single parameter which provides its user with an indication of the photometer's directional characteristics. Such a parameter is the specific acceptance angle which can be derived from the angular directional response curve. American National Standard General-Purpose Photographic Exposure Meters, PH 2.12, 4.2.3, defines the specific acceptance angle as "the angle in the stated directions from the optical axis of the receiver at which [a] point source must be placed to reduce the scale reading of the meter an amount corresponding to 50 percent of the original light reading when the same source was on the optical axis." From this definition, it is readily apparent that a photometer with a narrow specific acceptance angle, say on the order of a few degrees, would be classified as a "spot meter" convenient for measuring brightnesses of individual scene objects while a meter having a specific acceptance angle equivalent to, say one-half of a camera's field angle, would be characterized more as an "averaging" meter.
Again, these distinctions are important since they dictate how much importance will be placed on various scene objects in predicting exposures. What a photometer's specific acceptance angle should be depends on what kinds of scenes it will probably be used to measure. For a horizontal landscape in which the bright sky might tend to cause underexposure of the ground area, it generally is desirable to have a photometer weighted in favor of the ground in order to minimize problems with the sky. On the other hand, for an overall evenly lit scene, it is generally considered desirable to have a fairly broad specific acceptance angle so that no one object dominates; the theory being that a better average is obtained by including as much of the scene as possible. For anomolous scene conditions such as high contrast, back-lit scenes, it would be desirable to have a narrow specific acceptance angle which measured only the subject of interest. Because of the variety of scene conditions, it is clear that no one specific acceptance angle can satisfy the requirements for all scene conditions. Therefore a compromise is required which will result in the largest amount of well exposed pictures for the expected range of scene conditions. One solution which has been found satisfactory is to provide a photometer having varying specific acceptance angles, being generally broader in the horizontal plane than in the vertical plane, while simultaneously being aimed below the horizontal plane.
There are a number of prior art disclosures which are generally related to the foregoing solution. For example, U.S. Pat. No. 3,134,021 issued to M. Ploke on May 19, 1964 and entitled "PHOTOELECTRIC EXPOSURE METERS WITH LIGHT RAY LIMITERS" disclosed a photometer comprising a photocell in which a light ray limiter is positioned in front of a photocell for the purpose of reducing the acceptance angle in the vertical direction without changing the acceptance angle in the horizontal direction. U.S. Pat. No. 3,062,964 issued to Marvin Lubin on Nov. 6, 1962 and entitled "OPTICAL SYSTEMS FOR PHOTOCELLS" discloses an optical system by which the acceptance angle of a photocell is different in the vertical and horizontal directions. And in U.S. Pat. No. 2,248,758 issued to R. A. E. Higonnet et al. on July 8, 1941 and entitled "OPTICAL SYSTEM FOR REDUCING THE VIEWING ANGLE OF CERTAIN DEVICES SUCH AS PHOTOELECTRIC CELLS", the patentees disclose the use of prisms in an optical system for reducing the viewing angle of a photoelectric cell. According to this disclosure at column 2, lines 23-26, "the angles of the prisms mentioned above may be made different when it is desired to reduce the viewing angle to different values in different directions."
While the foregoing patents relate to either structure or optical arrangements for providing different horizontal and vertical "acceptance angles", meaning viewing angles, they do not teach, as the present invention does, how to selectively expand the specific acceptance angle of a photometer without appreciably altering the photometer's field of view.
Consequently, it is a primary object of this invention to provide an improved unicell photometer having different horizontal and vertical specific acceptance angles.
It is another object of this invention to provide an improved unicell photometer wherein the horizontal specific acceptance angle is broader than its vertical specific acceptance angle while the field of view in both directions is not appreciably different.
Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure.